<p><p><figure id='attachment_3407' style='max-width:975px' class='caption aligncenter'><img class="wp-image-3407 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Plan view of the areas of improvement for the widening the embankment of the I-95 and US Route 1 interchange in Alexandria, Virginia." width="975" height="552" /><figcaption class='caption-text'> Areas of ground improvement. (Lambrechts et al. 2003; With permission from ASCE)</figcaption></figure></p><p><div><h2>Project Summary/Scope:</h2>Widening of an embankment (doubling its width) along a 1.6-km stretch at the I-95/US Route 1 Interchange in Alexandria, Virginia.</p><p>Subsurface Conditions: Highly plastic, soft alluvial clays ranging from 2.5 to 7.5 meters (8 to 25 feet) in thickness with organic contents of 8 to 14%. The LL and PL of the clay averaged 80 and 25%, respectively. Clay near the existing embankment was slightly overconsolidated (OCR = 1.4 to 1.8) due to previous surcharge during the original construction. The soft clay is underlain by dense terrace sand deposits.</p><p>Large settlements were expected for new embankments, so wet DMM columns were selected to limit settlements and to avoid short-term stability problems. Project requirements also included maintaining existing traffic flow and meeting a tight construction schedule. Soil-cement buttresses (6 meters in length) and individual columns with 0.9-meter (3-foot) clear spacing and 23% area replacement ratio were used along the edge of the new embankments to resist unbalanced horizontal loads as well as to provide foundation support for a new 650-meter (2,000-foot) long MSE wall. High organic content in the clay required a cement dosage rate of 425 kg/m³ whereas the average rate in other areas was 300 kg/m³.</p><p>A 6-meter (20-foot) wide transition zone of more closely spaced columns with an area replacement ratio of 30% was used next to the buttresses. The SHANSEP approach was used to define an averaged shear strength of the organic clay where s_u= 0.25σ’_vcOCR^0.8. Global stability was analyzed using circular and irregular surfaces with XSTABL and UTEXAS3. The minimum factor of safety permitted was 1.3. A 0.9‑meter granular fill layer without geosynthetic was used on top of the deep mixed columns to transfer embankment loads to the columns.<br><h2>Complementary Technologies Used:</h2><strong> </strong>Prefabricated Vertical Drains (PVDs)<br><h2>Alternate Technologies:</h2>Technologies considered included lightweight fills, staged construction with prefabricated vertical drains and surcharge, excavation and replacement, and bridge structures in lieu of embankments.</p><p><figure id='attachment_3409' style='max-width:1024px' class='caption aligncenter'><img class="wp-image-3409 size-large" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Plan view and cross-section for typical SCCW and PV drain for widening the embankment of the I-95 and US Route 1 interchange in Alexandria, Virginia." width="1024" height="732" /><figcaption class='caption-text'> Soil-cement buttress and restrained embankment (Lambrechts et al. 2003).</figcaption></figure><h2>Performance Monitoring:</h2>Average 28-day compressive strength was set at 1,100 kPa (160 psi) with the lowest material strength allowed at 690 kPa (100 psi). The contractor was required to obtain the core samples and provide them to VDOT for testing.<br><h2>Cost Information:</h2>The Woodrow Wilson Bridge Reconstruction Project totaled $2.4 billion; the cost of the interchange on the Virginia side is estimated to be $350 million. The bid for 150,000 yd³ of deep mixing was $92 per cubic yard in 2001.<br><h2>Project Technical Papers:</h2>Lambrechts, J.R., Ganse, M.A., and Layhee, C.A., (2003), Soil mixing to stabilize organic clay for I-95 widening, Alexandria, VA., Proceedings of the 3rd International Conference on Grouting and Ground Treatment. <a href="http://ascelibrary.org/doi/abs/10.1061/40663%282003%2930">http://asceli…, J. R. and Layhee, C. A., (2003), Stability Analyses of Deep Soil Mix Buttresses in Organic Clay for I-95 Widening, Alexandria, VA, Proceedings 12th Panamerican Conference on Soil Mechanics and Geotechnical Engineering</p><p>Shiells, D., personal communication (August 2009)<br><h2>Date Case History Prepared:</h2>November 2012</p><p></div></p></p>